1. Field of the Invention
The present invention relates to a direct-current (D.C.) converter that is compact and inexpensive with high efficiency.
2. Description of Related Art
FIG. 1 shows a circuit diagram of a direct-current (D.C.) converter disclosed in Japanese Patent Publication Laid-open No. 2003-319650. In FIG. 1, this D.C. converter is formed by a half-bridge circuit. A series circuit consisting of a switching element Q1 (MOSFET) and a switching element Q2 (MOSFET) is connected to both ends of a direct-current (D.C.) power source Vin. In the switching element Q2, its drain is connected to a positive pole of the D.C. power source Vin. In the switching element Q1, its source of is connected to a negative pole of the D.C. power source Vin.
Between the source of the switching element Q1 and its drain, a diode D1 and a voltage resonant capacitor Cv are connected in parallel with the switching element Q1, respectively. Further, a series circuit including a reactor Lr, a primary winding P of a transformer T1 and a current-resonance capacitor Ci is connected to the drain and the source of the switching element Q1. The reactor Lr is formed by a leakage inductance between a primary side of the transformer T1 and its secondary side. A reactor Lp is connected to the primary winding P of the transformer T1 equivalently to its excitation inductance. Between the drain of the switching element Q2 and its source, a diode D2 is connected in parallel with the switching element Q2.
In the transformer T1, the secondary winding S has its one end (indicated with mark “●”) connected to an anode of a diode D3 and a cathode of a diode D4. The diode D3 has its cathode connected to one end of a smoothing capacitor C4. The diode D4 has its anode connected to the other end of the capacitor C4.
In the transformer T1, the other end of the secondary winding S is connected to an anode of a diode D5 and a cathode of a diode D6. The diode D5 has its cathode connected to one end of the capacitor C4. The diode D6 has its anode connected to the other end of the capacitor C4. A load RL is connected to both ends of the capacitor C4.
In order to make an output voltage Vo from the capacitor C4 constant, a control circuit 10 alternately turns on/off the switching element Q1 and the switching element Q2 on the basis of an output voltage Vo from the capacitor C4, accomplishing PFM (Pulse Frequency Modulation) control.
Referring to a timing chart of FIG. 2, we now describe the operation of the conventional D.C. converter in prior art, in detail.
In FIG. 2, Vds1 designates a voltage between the drain and the source of the switching element Q1, Id1 a drain current of the switching element Q1, ID1 a current of the diode D1, Vds2 a voltage between the drain and the source of the switching element Q2, Id2 a drain current of the switching element Q2, ID2 a current of the diode D2, VCv a voltage at both ends of the voltage resonant capacitor Cv, ICv a current of the voltage resonant capacitor Cv, ILr a current of the reactor Lr, ILp a current of the reactor Lp, VCi a voltage at both ends of the voltage resonant capacitor Ci, ID3 a current of the diode D3, and ID5 designates a current of the diode D5.
In the operation, it is noted that the switching element Q1 and the switching element Q2 are turned ON/OFF alternately to each other while containing a dead time when the switching element Q1 and the switching element Q2 are turned OFF together.
Within a period between t0 and t1, the state of the switching element Q1 is changed from ON-state to OFF state at t0. In a situation where the switching element Q1 is being turned ON, current flows in the route of Ci→Lp→Lr→Q1→Ci on the primary side of the transformer T1, while current flows in the route of C4→RL→C4 on the secondary side of the transformer T1. When the switching element Q1 is turned OFF, the current that had been flowing on the primary side of the transformer T1 is commutated from the switching element Q1 to the voltage resonant capacitor Cv, so that the current flows in the route of Ci→Lp→Lr→Cv→Ci.
As a result, the voltage resonant capacitor Cv is charged up to a voltage of the D.C. power source Vin although the voltage of the voltage resonant capacitor Cv had been exhibiting 0V while the switching element Q1 had been being turned ON. In connection, the voltage of the D.C. power source Vin will be also indicated with “Vin”, hereinafter. Thus, as the voltage VCv of the voltage resonant capacitor Cv is equal to the voltage Vds1 of the switching element Q1, the voltage Vds1 of the switching element Q1 rises from 0V to Vin. Correspondingly, the voltage Vds2 of the switching element Q2 falls from Vin to 0V as the voltage Vds2 of the switching element Q2 is equal to a difference of (Vin−VCv).
In the period between t1 and t2, when the voltage VCv of the voltage resonant capacitor Cv rises to Vin at time t1, the diode D2 becomes conductive, so that current flows in the route of Ci→Lp (P)→Lr→D2→Vin→Ci. Then, the voltage of the secondary winding S of the transformer T1 reaches the output voltage Vo, so that there arise two current flows in the route of C4→RL→C4 and the route of S→D3→C4→D6→S on the secondary side of the transformer T1. Further, since a gate signal for the switching element Q2 is outputted during the period between t1 and t2, the switching element Q2 carries out both zero-voltage switching (ZVS) operation and zero-current switching (ZVC) operation.
During the period between t2 and t3, there arises a current flow in the route of Vin→Q2→Lr→Lp (P)→Ci→Vin since the switching element Q2 has been turned ON at t2, so that the voltage VCi of the capacitor Ci rises with time. Then, on the secondary side of the transformer T1, there arise two current flows in the route of S→D3→C4→D6→S and the route of C4→RL→C4. It is noted that the voltage of the secondary winding S is clamped at the output voltage Vo, while the voltage of the primary winding P is clamped at a voltage in the turn ratio of the transformer T1 to the output voltage Vo. Therefore, resonant current due to the reactor Lr and the current resonant capacitor Ci is flowing on the primary side of the transformer T1.
In the period between t3 and t4, as the voltage of the secondary winding S becomes less than the output voltage Vo at t3, there arises a current flow in the route of C4→RL→C4 on the secondary side of the transformer T1. While, on the primary side of the transformer T1, the current flows in the route of Vin→Q2→Lr→Lp→Ci→Vin. That is, on the primary side of the transformer T1, there arises a flow of resonant current by the sum (Lr+Lp) of two reactors Lr, Lp and the current resonant capacitor Ci.
In the period from between t4 and t5, when the switching element Q2 is turned OFF at t4, the current flowing on the primary side of the transformer T1 is commutated from the switching element Q2 to the voltage resonant capacitor Cv, so that the current flows in the route of Lr→Lp→Ci→Cv→Lr.
Accordingly, the voltage resonant capacitor Cv, whose voltage has been equal to approx. Vin while the switching element Q2 is being turned ON, is discharged to approx. 0V. Thus, as the voltage VCv of the voltage resonant capacitor Cv is equal to the voltage Vds1 of the switching element Q1, the same voltage Vds1 falls from Vin to 0V. Correspondingly, the voltage Vds2 of the switching element Q2 rises from 0V to Vin as the voltage Vds2 of the switching element Q2 is equal to a difference of (Vin−VCv).
In the period between t5 and t6, when the voltage VCv of the voltage resonant capacitor Cv falls to 0V at t5, the diode D1 becomes conductive, so that current flows in the route of Lr→Lp (P)→Ci→D1→Lr. Then, the voltage of the secondary winding S of the transformer T1 reaches the output voltage Vo, so that there arise two current flows in the route of C4→RL→C4 and the route of S→D5→C4→D4→S on the secondary side of the transformer T1. Further, since a gate signal for the switching element Q1 is outputted during the period between t5 and t6, the switching element Q1 carries out both zero-voltage switching (ZVS) operation and zero-current switching (ZVC) operation.
During the period between t6 and t7, there arises a current flow in the route of Ci→Lp (P)→Lr→Q1→Ci since the switching element Q1 has been turned ON at t6, so that the voltage VCi of the capacitor Ci decreases with time. On the other hand, on the secondary side of the transformer T1, there are two current flows in the route of S→D5→C4→D4→S and the route of C4→RL→C4. The voltage of the secondary winding S is clamped at the output voltage Vo, while the voltage of the primary winding P is clamped at a voltage in the turn ratio of the transformer T1 to the output voltage Vo. Thus, resonant current due to the reactor Lr and the current resonant capacitor Ci is flowing on the primary side of the transformer T1.
In the period between t7 and t8, as the voltage of the secondary winding S becomes less than the output voltage Vo at t7, there arises a current flow in the route of C4→RL→C4 on the secondary side of the transformer T1. While, on the primary side of the transformer T1, the current flows in the route of Ci→Lp→Lr→Q1→Ci. That is, on the primary side of the transformer T1, there arises a flow of resonant current by the sum (Lr+Lp) of two reactors Lr, Lp and the current resonant capacitor Ci.